Calcium paradox-evoked release of prostacyclin and immunoreactive leukotriene C4 from rat and guinea-pig hearts. Evidence that endogenous prostaglandins inhibit leukotriene biosynthesis.

The purpose of this study was to assess the influence of the calcium paradox (5 min calcium-free perfusion followed by 15 min calcium repletion) on the release of immunoreactive leukotriene C4 and 6 Keto-prostaglandin F1 alpha from rat and guinea-pig hearts. Under control conditions or during the 5 min calcium-free perfusion period no immunoreactive leukotriene C4 was detectable in the coronary effluent. Following reperfusion with calcium-containing medium a large release of leukotriene C4 was observed although the amount was significantly greater in the rat heart. 6 keto-prostaglandin F1 alpha was detected during normal and calcium-free perfusion and the release was significantly stimulated during calcium repletion. Treatment with ibuprofen, a cyclo-oxygenase inhibitor, prevented the release of 6 keto-prostaglandin F1 alpha but increased the efflux of immunoreactive LTC4 during calcium repletion. Arachidonic acid, the substrate for prostaglandin and leukotriene synthesis increased the efflux of 6 keto-prostaglandin F1 alpha but decreased the release of leukotriene C4. The latter effect was reversed by perfusion with ibuprofen, and mimicked by prostacyclin, the primary cardiac prostaglandin. This study shows that the calcium paradox is a potent stimulus for eicosanoid release from rat and guinea-pig hearts, a phenomenon likely due to the activation of calcium-dependent enzymes. The study also suggests that endogenous prostaglandins inhibit leukotriene synthesis in cardiac tissue.